There is known an electromagnetic flowmeter comprising an exciting coil to create a magnetic field orthogonal to the direction of fluid flow running inside a measurement tube and signal electrodes provided inside the measurement tube. In such a device, the electromotive force generated across the electrodes by the flow of fluid through the measurement tube can be used to determine the fluid flow rate.
The essential parts of a conventional electromagnetic flowmeter are indicated in FIG. 8. In this diagram, a measurement tube 1 comprises a non-magnetic metal pipe 2 (for example, non-magnetic stainless steel pipe) and a lining 3 based on an insulative resin formed on the inside of the non-magnetic metal pipe 2. 4a and 4b are facing signal electrodes provided on the inner peripheral surface of the measurement tube 1.
Further, although not indicated in FIG. 8, an exciting coil that creates a magnetic field is provided orthogonally to the direction of flow of the fluid that flows inside the measuring tube 1, and the facing signal electrodes 4a and 4b are provided orthogonally to the magnetic field that this exciting coil creates (see, e.g., Japanese examined patent application No. 7-9374 of Hei et al., hereinafter “Hei.”)
As indicated in Hei, this electromagnetic flowmeter comprises an AC synchronous circuit that generates pulse signals synchronized to the frequency of the commercial power source. The exciting current supplied to the exciting coil and the rate at which flow rate signals are sampled between the signal electrodes are both clocked by the pulse signals generated by this AC synchronizing circuit.
FIG. 9 illustrates the configuration of a circuit that includes the AC synchronizing circuit described in Hei. In this diagram, 5 is a power source circuit, 6 is a dividing circuit, 7 is a comparator, 8 is an even-number frequency divider and 9 is a timing circuit. The AC synchronizing circuit 10 is composed of the power source circuit 5, the dividing circuit 6 and the comparator 7. In this circuit configuration, a single threshold value Vth is set up in the comparator 7. The pulse signal, which the AC synchronizing circuit 10 generates and which is synchronized to the frequency of the commercial power source, will be referred to as the “synchronized pulse signal” throughout the remainder of this application.
FIGS. 10(a)-10(e) illustrate the signal waveforms for the circuit configuration in FIG. 9. In the circuit configuration indicated in FIG. 9, AC signals input from the commercial power source are sent to the dividing circuit 6 through the power source circuit 5, where they are divided by the dividing circuit 6 and made into divided AC signals (FIG. 10(a)). The divided AC signals are sent to the input of the comparator 7. The comparator 7 monitors the level of the divided AC signals. If the level of a divided AC signal exceeds the threshold value Vth, that signal is inverted from the “L” level to the “H” level. If the level of a divided AC signal falls below the threshold value Vth, that signal is inverted from the “H” level to the “L” level. A synchronized pulse signal like that indicated in FIG. 10(b) can thereby be obtained as an output signal from the comparator 7.
Some of these synchronized pulse signals are directly output to the timing circuit 9, and others are frequency-divided in even-number multiples by the even-number frequency dividing circuit 8, and are then output to the timing circuit 9. As a result, as indicated in FIG. 10(c), the frequency-divided synchronized pulse signals from the even-number frequency dividing circuit 8 are output unchanged as first timing signals from a terminal X of the timing circuit 9, and the direction of the exciting current to the exciting coil is switched based on these first timing signals. Moreover, signals like those indicated in FIG. 10(d) are output from a terminal Y of the timing circuit 9 as second timing signals; signals like those indicated in FIG. 10(c) are output from a terminal Z of the timing circuit 9 as third timing signals; and flow rate signals produced between the signal electrodes 4 are sampled based on these second and third timing signals.
Nonetheless, in the conventional electromagnetic flowmeter described above, momentary power source voltage fluctuations are generated by the AC signals input from the commercial power source. For example, as indicated by the code Pz in FIG. 11(a), if the level of the divided AC signal sent to the comparator 7 momentarily changes up and down while at the threshold value Vth, this change is judged to be one waveform of the divided AC signal, and an extra pulse number is generated (refer to FIG. 11(b)). Therefore, the frequency of the synchronized pulse signal will fluctuate. In this case, the first to third timing signals, which take the synchronized pulse signals as a standard, also fluctuate, and a discrepancy is generated between the timing for switching the direction of the exciting current to be supplied to the exciting coil and the timing for sampling the flow rate signals generated between the signal electrodes 4. As a result, the device cannot render accurate flow rate measurements.
Therefore, there is a need for electromagnetic flowmeters that can accurately measure flow rate without producing fluctuations in the frequency of the synchronized pulse signals.